Tensions have risen across eastern Ukraine after authorities in Kyiv launched a military offensive early on May 2 against pro-Russian separatists who seized control of government buildings in the eastern city of Slovyansk.

Russia, which has deployed an estimated 40,000 soldiers on its border with Ukraine, says Kyiv's use of military force "against its own people" in eastern Ukraine will lead to a "catastrophe."

Ukrainian Interior Minister Arsen Avakov said the goals of the offensive were to get pro-Russian separatist forces to "free the hostages, lay down their arms, and free administrative buildings, and restore the normal functioning of the town's administration."

He also described the pro-Russian separatist forces as professional mercenaries.

At least three people were killed on May 2 in the battle for control of checkpoints on the outskirts of Slovyansk -- including two Ukrainian pilots whose helicopters were shot down and at least one pro-Russian fighter.

A spokeswoman for separatist forces in Slovyansk told RFE/RL that more separatists had been killed but did not provide a number.

The Ukrainian Security Service (SBU) said a third Ukrainian pilot was taken prisoner by the separatists in Slovyansk, a city of about 140,000.

After about four hours of fighting that began just before dawn, the advance by government forces slowed.

Government forces had captured at least nine checkpoints that had been erected by separatist fighters and seized other strategic positions around the city by 8 a.m. local time.

But closer to the city center, separatist forces continued to block roads and rail lines linking Slovyansk with other towns in eastern Ukraine.

Serhiy Lyleyev, head of the local branch of Ukrainian nationalist Svoboda party, told RFE/RL later on May 2 that he had seen some ground troops from the Ukrainian Army deployed in parts of Slovyansk.

But the separatists continued to hold key government buildings in the city center.

WATCH: Angry villagers try to block Ukrainian troops in armored personnel carriers as they move into the village of Andriivka in the country's east.

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In Donetsk, where pro-Russian separatists on May 1 seized the local prosecutor's office and fortified their control of other government buildings, separatist forces responded to news of the Slovyansk offensive by seizing the regional control center for Donetsk railways in eastern Ukraine.

Reuters quoted a spokesman for the Donetsk railway as saying the separatists cut the electricity supplies needed to keep railroad cars moving in the region.

But in the eastern city of Luhansk, where pro-Russian separatists on April 29 also seized control of government and police buildings, the Interior Ministry said separatists met with local authorities after the Slovyansk offensive began and agreed to move out of the prosecutor's office and a local television center.

In Ukraine's southern port city of Odesa, both Ukrainian and Russian television were reporting clashes between supporters of the Kyiv government and pro-Russian separatists.

'Growing Concern'

The European Commission has said it's watching the events in eastern Ukraine with growing concern.

The rebels have detained Slovyansk's mayor, several journalists, three SBU agents, and are also holding a team of OSCE military observers.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, called the Ukrainian assault a "reprisal raid" and said "peaceful settlements" were being shelled by warplanes. Peskov said the operation was destroying the "last hope for viability of the Geneva accords."

Peskov said the Kremlin had sent presidential envoy Vladimir Lukin to Ukraine's southeast to negotiate the release of the OSCE monitors.

Russia's Foreign Ministry also accused the European Union of destroying chances for a peaceful resolution to Ukraine's crisis by supporting those that Moscow says were behind a "coup" in Kyiv.

It demanded that the West renounce what Moscow calls "its destructive policies toward Ukraine."

Hagel: 'Clarifying Moment' For NATO

In Washington, U.S. Defense Secretary Check Hagel said that Russia's aggression in Ukraine presented a "clarifying moment" for the NATO alliance.

"Russia's actions in Ukraine have made NATO's value abundantly clear," Hagel said in remarks at the Wilson Center in Washington.

Hagel called for a "renewed financial commitments" from all NATO members.

The U.S. defense secretary said the European members of NATO need to boost their defense investment because over the long run Russia will test the purpose, stamina, and commitment of the U.S.-led, 28-member alliance.

"We must not squander this opportunity or shrink from this challenge. We will be judged harshly if we do," he said.

In Berlin, European Union foreign-policy chief Catherine Ashton said it was "absolutely vital" that the occupation of government buildings by separatists in Ukraine be reduced and that Ukraine's parliament be viewed as a "place where all groups can consult."

In Kyiv, acting Ukrainian President Oleksandr Turchynov has signed an order reinstating military conscription. The order comes into effect immediately.

In Israel, the mayor of the eastern Ukrainian city of Kharkiv, Hennadiy Kernes, has regained consciousness.

Kernes was flown to Israel after being shot by unknown assailants on April 28.

Kernes first held a pro-Russian stance, but later positioned himself as loyal to the pro-Western government in Kyiv.